This invention relates to ignition devices for internal combustion engines, and more particularly to ignition devices for multi-cylinder internal combustion engines which are provided with the function for detecting the operation of the ignition coils for the respective cylinders of the engine.
FIG. 3 is a circuit diagram showing a conventional ignition device for an internal combustion engine. In FIG. 3, a driver transistor 2 is coupled across a battery 1 via a resistor 3. A power transistor 4 in Darlington connection is driven by the output of the driver transistor 2. The ignition coil 5 has a primary coil coupled across the positive terminal of the battery 1 and the collector of the power transistor 4. The secondary coil of the ignition coil 5 is coupled to the ignition plug (not shown) via a distributor (not shown). A current detecting resistor 6 is coupled across the emitter of the power transistor 4 and the ground. In response to the voltage developed across the current detecting resistor 6, a current limiter circuit 7 controls the base current of the power transistor 4, and thereby limits the current flowing through the primary coil of the ignition coil 5. An ignition detector circuit 8 detects the voltage developed at the collector of the power transistor 4 upon interruption of the current flowing through the primary coil of the ignition coil 5, and generates an ignition confirmation signal. In the figure, the ignition device is enclosed within a dot-and-dash line box 9.
The operation of the conventional ignition device of FIG. 3 is as follows.
When the driver transistor 2 is turned off in response to an external signal, the power transistor 4 is turned on to let the current flow through the primary coil of the ignition coil 5. Under this circumstance, a voltage corresponding to the current flowing through the primary coil of the ignition coil 5 is developed across the current detecting resistor 6. This voltage developed across the current detecting resistor 6 is supplied to the current limiter circuit 7. In response to this voltage, the current limiter circuit 7 controls the base current of the power transistor 4, and thereby limits the current through the primary coil of the ignition coil 5 to a predetermined magnitude, such that the current through the primary coil of the ignition coil 5 is maintained at a constant level.
Then, when the driver transistor 2 is turned on by an ignition signal and the power transistor 4 is turned off to interrupt the current through the primary coil of the ignition coil 5, a high voltage is developed across the secondary coil of the ignition coil 5 and is supplied to the ignition plug of the engine via the distributor.
The ignition detector circuit 8 detects the voltage developed at the collector of the power transistor 4 upon interruption of the current flowing through the primary coil of the ignition coil 5. When the detected voltage exceeds a predetermined level, the ignition detector circuit 8 generates an ignition confirmation signal. The ignition confirmation signal is supplied to a computer (not shown), such that the computer may judge whether or not the ignition coil is functioning properly. The computer determines to which cylinder of the engine the malfunctioning ignition coil corresponds, and the fuel supply to the malfunctioning cylinder is stopped in a fuel equalization system.
FIG. 4 is a circuit diagram showing another conventional ignition device for an internal combustion engine. The detector input of the ignition detector circuit 8 is coupled to the current detecting resistor 6, such that the voltage corresponding to the current flowing through the primary coil of the ignition coil 5 can be detected. Thus, the ignition detector circuit 8 detects the voltage developed across the current detecting resistor 6 corresponding to the current flowing through the primary coil of the ignition coil 5, and generates an ignition confirmation signal when the detected current level exceeds a predetermined level. Otherwise the circuit of FIG. 4 is similar to that of FIG. 3.
The above conventional ignition devices, however, have the following disadvantage. When a plurality of ignition device units are provided for respective cylinders of a multi-cylinder and the ignition coil and the power transistor of each unit are assembled into an integral unit, separate ignition detector circuits must be provided for respective cylinders. Thus, the circuit becomes complicated and expensive.